1. Field of the Invention
The present invention relates to an orthogonal converting apparatus which is used for a high efficient encoding of a video signal or the like.
2. Related Background Art
Hitherto, as a method of high efficient encoding an image or a voice, an orthogonal converting process such as a discrete cosine transformation (DCT) or the like has been known.
FIG. 1 shows a two-dimensional DCT processing circuit for an input signal comprising a block of (8.times.8) pixels. The DCT processing circuit is constructed by two one-dimensional DCT circuits for eight input signals and a transposition circuit. Since a processing time of the one-dimensional DCT circuit is especially long, some high speed algorithms have been considered. FIG. 2 shows an example of a one-dimensional DCT using a known high speed algorithm. In such a DCT, a symmetry of a transforming equation of the DCT is utilized, the addition and subtraction are preliminarily symmetrically executed to input signals and the results are combined, thereby reducing the number of multiplications to half of the number.
A high speed algorithm using a butterfly method such as an FFT is also widely known. In this algorithm, common portions included in the transforming equation of the DCT are combined, thereby further reducing the number of multiplications. Since many of the apparatuses using the high speed algorithm have, however, a large circuit scale, it has been pointed out that they are unsuitable for an application to formation of an LSI or the like.
A system in which the signal converted by the DCT is weighted by using visual sense characteristics such that the human eyesight is dull to high frequencies is also generally used. In such a system, an encoding efficiency is often intended to be raised by largely weighting a signal indicative of low frequencies and slightly weighting a signal indicative of high frequencies. FIGS. 3A and 3B show examples of a 2-dimensional DCT in which a weighting process is performed. FIG. 3A shows the example in which the weighting process is executed after the ordinary 2-dimensional DCT. FIG. 3B shows the example in which the weighting process is executed every 1-dimensional DCT. A method of using both of the DCT and multipliers for weighting is well known and is shown in Japanese Patent Laid-Open Application No. 2-116969.
On the other hand, in a highly efficient encoding of a TV video signal or the like in which an input signal is constructed by two fields having a time deviation, an idea such that a motion between the fields is detected and a method of the DCT is switched is widely used. FIG. 4 shows one example of such a method. Before executing a DCT process, a motion information signal is supplied from a motion detection circuit (not shown) to a DCT circuit, and in accordance with the presence or absence of the motion, a DCT method is switched to either one of a method of performing an ordinary 2-dimensional DCT to (8.times.8) input signals and a method of dividing the input signals into two sets of (4.times.8) input signals comprising the addition and subtraction between the fields and performing the 2-dimensional DCT to each set.
In many DCT circuits, as mentioned above, there is a problem of an increase in circuit scale because of the idea of improving the high processing speed and high efficiency. Especially, in an application such that both of the DCT and an IDCT which is an inverse transformation of the DCT are necessary, there is a problem such that the circuit scale further increases and it is difficult to form an LSI.